Atrial fibrillation, atrial flutter and paroxysmal supraventricular tachycardia are common and important supraventricular arrhythmias in man; these three arrhythmias account for almost 50 percent of U.S. hospitalizations for arrhythmias. The purpose of this protocol is to conduct studies that will improve our overall understanding of the behavior of symptomatic and asymptomatic paroxysmal supraventricular arrhythmias with the long-term goal of developing improved antiarrhythmic therapies. Adult patients with paroxysmal supraventricular tachycardia and atrial fibrillation will be recruited. Transtelephonic electrocardiogram monitoring will be used to document symptomatic arrhythmias, and 24-hour continuous ambulatory electrocardiogram monitoring (Holter monitoring) will be used to document asymptomatic arrhythmias. When patients record and transmit symptomatic arrhythmias, a telephone contact will be made within 24 hours to obtain the symptoms that occurred and the time that the symptoms began. The occurrence of symptomatic and asymptomatic arrhythmias will be compared. The occurrence of arrhythmias during antiarrhythmic drug treatment periods will be compared with the occurrence during untreated periods to assess features of successful antiarrhythmic drug efficacy in individual patients. This study is important because it will lead to a better understanding of successful antiarrhythmic drug therapy and a improved methods to test antiarrhythmic drug therapy for supraventricular arrhythmias including atrial fibrillation, atrial flutter and paroxysmal supraventricular tachycardia. Atrial fibrillation alone appears to affect almost 2,000,000 people in the United States; and hospitalization for atrial fibrillation is the leading cause of hospitalization for arrhythmias. Patients with atrial fibrillation have a five-fold increased risk of stroke and two-fold increased risk of cardiovascular mortality. Studies during the next year will focus particularly on the relationship of antiarrhythmic drug therapy to the occurrence of symptomatic and asymptomatic arrhythmias and on the relationship of 24-hour clock time that antiarrhythmic drugs are taken and 24-hour clock time that symptomatic arrhythmias recur.